The present invention relates to a patellar implant and to a total or partial knee prosthesis incorporating such an implant.
In the domain of knee prostheses, it is known to fit a natural patella with an implant intended to form one or more articular surfaces provided to cooperate with the femoral articular surfaces, whether it be question of natural surfaces or, most often, of prosthetic surfaces.
It is known, for example from U.S. Pat. No. 5,609,640 or from EP-A-0 021 421, to produce a patellar implant in the shape of a spherical dome or mushroom. Taking into account its geometry, such an implant is in point or virtually point contact with each condyle, which induces considerable localized stresses in the implant, on the one hand, and in the femoral prosthetic component or the femur, on the other hand. It is also known from FR-A-2 700 260 to produce a patellar component with two articular surfaces, of spherical and concave shape, which are symmetrical with respect to the sagittal plane and separated by a joining surface likewise symmetrical with respect to this plane. Taking into account the geometry of the Joining surface, the articular surfaces can generally not be in surface abutment against the trochlea during a whole movement of flexion or of extension of the knee.
This imperfect congruence between the articular surfaces provided respectively on the patellar implant and on the femur also results in a medio-lateral instability of the patella, which may lead to an unnatural positioning of the patella. This lack of congruence also induces an instability of the patella when pivoting in the sagittal plane, this reducing the efficiency of the extensor muscle and tending to increase wear of the articular surfaces in contact. Taking these limitations into account, such an implant must be fitted very precisely, otherwise it cannot perform its function.
It is a particular object of the present invention to overcome these drawbacks by proposing a patellar implant whose geometry is adapted to allow an efficient bearing on the trochlea of the femur, such bearing remaining congruent in the different positions of the knee joint.
To that end, the present invention relates to an implant of the aforementioned type, which comprises an outer articular surface and an inner articular surface provided to cooperate respectively with an outer side and an inner side of a femoral trochlea or of a femoral prosthetic component and joined together by a transition ridge, while the outer articular surface is concave in a plane parallel to the sagittal plane of the joint and in a transverse plane. This implant is characterized in that the transition ridge is curved as viewed from the front of the implant, with a concavity facing the outer articular surface.
Within the framework of the present invention, the adjective xe2x80x9cinnerxe2x80x9d corresponds to that side of a knee joint oriented towards the other knee, while the adjective xe2x80x9couterxe2x80x9d corresponds to the side facing away from the other knee. The sagittal plane is a plane intersecting the knee from rear to front in a median zone, while a transverse plane is a plane perpendicular to the patellar tendons and to the tendons of the quadriceps.
Thanks to the invention, the congruence of the articular surfaces provided on the implant and the trochlea is widely improved with respect to the state of the art. In particular, the concavity of the outer articular surface in the two planes perpendicular with respect to each other makes it possible to increase the stability of the contact and to reduce the wear of this surface. This double concavity, possibly associated with the convexity of the inner articular surface, also makes it possible to adjust the patella by a pivoting about the outer side of the femur, with the result that the implant and the patella are permanently correctly positioned with respect to the condyles and to the trochlea, without loss of congruence. The curved nature of the ridge contributes to the definition of the outer articular surface and is compatible with the afore-mentioned movement of adjustment, this ridge being engaged in the trochlea. It is a deliberate object of the invention to promote the congruence of the outer articular surfaces respectively provided on the implant and at the level of the outer side of the trochlea or of the outer condyle, whether it be question of a natural condyle or of a prosthetic element, as the bearing efforts of the patella on the condyles essentially transit via these outer surfaces.
According to advantageous but non-obligatory aspects of the invention, the implant incorporates one or more of the following characteristics:
The centre of curvature of the transition ridge is located outside or on the periphery of the implant, seen from front view. This corresponds to the fact that the radius of curvature of this ridge is relatively large.
The outer articular surface of the implant is a surface of revolution.
Seen from front view, the implant is substantially ovoidal in shape.
The inner articular surface of the implant is planar or convex in a plane parallel to the sagittal plane. The fact that this surface is planar or convex in a plane parallel to the sagittal plane and that it offers less congruence is less favourable than the situation relative to the outer articular surface, which is not prohibitive as the efforts transiting via this surface are less than those transiting via the outer articular surface. Moreover, the planar or convex nature of this surface in such a plane allows this surface to slide on the inner condyle during a movement of adjustment of the implant, by pivoting of its outer articular surface on the outer condyle.
Two lateral bevels extend in directions substantially perpendicular to the sagittal plane and join the respective ridges of the articular surfaces to a peripheral edge of the implant. These bevels avoid a possible blocking of the implant on a corner or an edge of a femoral prosthetic surface.
The implant is composed of a one-piece plastics component which allows it to have a sufficient thickness, without risk of metal/metal contact between the femoral prosthetic element and a reinforcing plate such as exists in an implant made of two materials.
The invention also relates to a total or partial prothesis of the knee which comprises an implant as described hereinabove. Such a prosthesis is easier for the surgeon to implant, insofar as the implant may be positioned with a greater tolerance, the implant tending to adjust its position with respect to the condyle by the congruence of its outer articular surface and of the outer condyle. Such a prosthesis also tends to wear out less than the known prostheses.